In one type of known PEM fuel cell, the reactant gas flow field channels are in “separator” plates (typically called “water transport plates”) which also have water channels in the surface of one or both plates opposite to the reactant gas flow channels. The porous and hydrophilic plates allow product water to migrate from the cathode to the water channels, and allow water to migrate from the water channels to the anode, to assure uniform humidification. Cooling of the fuel cell is typically accomplished with the water in the water channels, which is circulated externally of the fuel cell stack to a heat exchanger that is selectively bypassed in order to maintain proper water temperature. Cooler plates may also be used.
When a fuel cell power plant is utilized within an electric vehicle, it is subject to being left inoperative in environments which can reach temperatures below the freezing temperature of water. External water plumbing, pumps, and accumulators are therefore subject to damage from expansion as ice is formed, or otherwise require extensive measures to drain and restore the water, to maintain the temperature above freezing, or to otherwise protect the apparatus from freeze damage.
In vehicles, space and weight are both at a premium. External water management components, such as pumps and accumulators, add to the weight and take up space.
A recent innovation disclosed in copending PCT patent application Ser. No. US 05/13040), filed Apr. 15, 2005, employs evaporative cooling in which water from the cathode is evaporated into the cathode air flow, and condensed at the air exit; the condensate is returned to the cells by means of minute water channels or wicking within or between the water transport plates. However, the need to provide even only a few, minute water channels (or equivalent porous wicking material between the cathode and the anode) between the two electrodes requires the manufacture of two plates, which because of having channels on both sides of at least one of them, requires a certain minimum thickness for physical integrity.